The present invention relates to a self-luminous display apparatus which includes a plurality of self-luminous pixels and a DC/DC (DC to DC: direct current to direct current) converter which can be used as a power supply for a self-luminous display apparatus of the type described and particularly to a DC/DC converter of the chopper type with which the switching loss is comparatively low.
A DC/DC converter is conventionally used for various electronic equipments because it is convenient to step down or step up the voltage of a DC power supply. Particularly, a DC/DC converter of the chopper type wherein a switching element is controlled on and off to obtain a predetermined voltage is spread widely because it is advantageous in that it includes a comparatively small number of parts and is small in size, light in weight and low in cost.
Meanwhile, it is demanded that electronic apparatus in recent years be small in size and convenient for carrying. Therefore, a battery is used frequently as a power supply for the apparatus. Therefore, a DC/DC converter which can step up the voltage is used frequently for a power supply circuit provided in the apparatus. Also where a self-luminous display unit is used in a terminal of the battery driven type such as, for example, a portable telephone set, usually a DC/DC converter of the voltage step-up type is used in order to supply a sufficient voltage to a driver circuit.
Here, an example of a DC/DC converter conventionally used as a power supply circuit for stepping up the voltage is shown in a circuit diagram of FIG. 3. Referring to FIG. 3, a choke coil L1xe2x80x2 and a switching transistor Q1xe2x80x2 are connected in series between an input terminal 1xe2x80x2 of the DC/DC converter and the ground, and a capacitor C1xe2x80x2 for filtering is connected between the input terminal 1xe2x80x2 and the ground. A node between the choke coil L1xe2x80x2 and the collector of the switching transistor Q1xe2x80x2 is connected to an output terminal 2xe2x80x2 of the DC/DC converter through a rectifying diode D1xe2x80x2, and a smoothing capacitor C2xe2x80x2 is connected between the output terminal 2xe2x80x2 and the ground. A pair of resistors R1xe2x80x2 and R2xe2x80x2 for voltage detection are connected in series between the output terminal 2xe2x80x2 and the ground, and a node between the resistors R1xe2x80x2 and R2xe2x80x2 is connected to an input terminal of a control circuit 3xe2x80x2. An output (drive signal) of the control circuit 3xe2x80x2 is connected to the base of the switching transistor Q1xe2x80x2.
In the circuit having the configuration described above, if an input voltage is first supplied from a DC power supply such as an external battery (not shown) to the input terminal 1xe2x80x2, then the control circuit 3xe2x80x2 supplies a drive signal to the switching transistor Q1xe2x80x2. Consequently, the switching transistor Q1xe2x80x2 repeats on and off operations to interrupt the current between the collector and the emitter of the switching transistor Q1xe2x80x2. While the switching transistor Q1xe2x80x2 is in an on state, current flows through the choke coil L1xe2x80x2 and the choke coil L1xe2x80x2 stores energy, but if the switching transistor Q1xe2x80x2 is placed into an off state, then the choke coil L1xe2x80x2 releases the energy in the form of a fly-back voltage. Thereupon, a high voltage formed from the fly-back voltage superposed on the input voltage appears at the node between the choke coil L1xe2x80x2 and the switching transistor Q1xe2x80x2. The superposed voltage of the two voltages passes through the rectifying diode D1xe2x80x2 and charges the smoothing capacitor C2xe2x80x2. A voltage appearing between the terminals of the smoothing capacitor C2xe2x80x2 in this manner servers as an output voltage of the power supply circuit and is supplied to an external load through the output terminal 2xe2x80x2. The output voltage is kept substantially fixed because the control circuit 3xe2x80x2 supervises the divided voltage appearing at the node between the resistors R1xe2x80x2 and R2xe2x80x2 and varies the on-duty of the switching transistor Q1xe2x80x2 in response to the divided voltage.
It is known that the conversion efficiency of the DC/DC converter relies much upon the inductance value of the choke coil L1xe2x80x2. FIG. 4 is a graph illustrating a relationship between the magnitude of the inductance value of the choke coil L1xe2x80x2 which is a component of the DC/DC converter and the efficiency of the DC/DC converter. When the output current of the DC/DC converter has a predetermined value Ic, the efficiency varies in response to the inductance value of the coil and exhibits a maximum efficiency at an inductance value Lc in FIG. 4. When a DC/DC converter is designed, a single choke coil having an inductance value with which the conversion efficiency is in the maximum with regard to predetermined output current corresponding to a steady state of the apparatus is determined.
A DC/DC converter developed from the circuit configuration of the type described is disclosed, for example, in Japanese Patent Laid-Open No. 252901/1999, Japanese Patent Laid-Open No. 262250/1999, Japanese Patent Laid-Open No. 2000-152611 and so forth.
FIG. 5 shows an example of a conventional DC/DC converter having a configuration different from those of the documents mentioned above. Referring to FIG. 5, one of terminals of a channel of a switching field effect transistor (FET) 1A is connected to an input terminal Vin of the DC/DC converter, and the cathode of a rectifying diode 2A and a choke coil 3A are connected to the other terminal of the switching FET 1A. The anode of the diode 2A is grounded, and the other terminal of the choke coil 3A is connected to an output terminal Vout of the DC/DC converter which in turn is connected to a load (not shown). The output terminal Vout is grounded through a smoothing capacitor 4A and connected to an input of an oscillation control circuit 5A. An output of the oscillation control circuit 5A is inputted to an oscillation circuit 6A, and an output (drive signal) of the oscillation circuit 6A which is controlled with the input from the oscillation control circuit 5A is connected to the gate terminal of the switching FET 1A.
The oscillation circuit 6A switches the switching FET 1A in a predetermined period. For such switching, the oscillation control circuit 5A controls the on-time of the switching FET 1A, that is, the pulse width to be applied to the gate of the switching FET 1A, so that the output voltage of the DC/DC converter may be fixed. During the on time of the FET 1A, power is stored into the choke coil 3A, but when the switching FET 1A is off, the power stored in the choke coil 3A is transmitted to the load through the rectifying diode 2A.
The output voltage of the DC/DC converter is kept fixed by controlling the on-time t of the switching FET 1A in this manner. The inductance value of the choke coil 3A is set to a value with which a stabilized oscillation operation of the oscillation circuit 6A can be achieved. A DC/DC converter developed from the circuit configuration described above is disclosed, for example, in Japanese Patent Laid-Open No. 294269/1996, Japanese Patent Laid-Open No. 121534/1997 and so forth.
Japanese Patent Laid-Open No. 121534/1997 mentioned above, particularly in FIG. 1 thereof, discloses a DC/DC converter wherein, in order to prevent such a disadvantage that, when the load current is low, a switching FET repeats intermittent oscillations, the load current is detected and the inductance value of a choke coil is varied based on the load current. FIG. 6 shows a circuit diagram of the DC/DC converter disclosed in the document mentioned above. In FIG. 6, as reference characters, reference characters used in the document are used with the suffix B added thereto so as to facilitate recognition of a corresponding relationship.
The DC/DC converter of FIG. 6 has a basic configuration similar to that of FIG. 5. Referring to FIG. 6, the DC/DC converter shown includes a switching FET 1B, a rectifying diode 2B for transmitting power stored in a choke coil to a load when the switching FET 1B is off, a smoothing capacitor 7B, and a PWM control section 10B for controlling the on-time of the switching FET 1B, that is, the pulse width to be applied to the gate of the switching FET 1B, so that an output voltage may be fixed. The DC/DC converter includes, in addition to the components mentioned above, two choke coils 3B and 4B connected in series, a pair of switches 5B and 6B connected in parallel to the choke coils 3B and 4B, respectively, a current detection section 8B for detecting load current, and a changeover control section 9B for switching on/off the switches 5B and 6B in response to the load current to equivalently vary the entire inductance value of the choke coils 3B and 4B. In the DC/DC converter, the load current is actually detected to selectively short-circuit the choke coils connected in series to stabilize the output voltage control when the load is low. Detailed description is omitted here because they are described in Japanese Patent Laid-Open No. 121534/1997 mentioned above.
By the way, in electronic equipments in recent years, a display unit wherein a large number of display elements are arranged in a matrix is used popularly because it has a high degree of freedom in display information, and the elements are selectively driven by a column driver and a row driver to display desired information. Such a configuration as just described is hereinafter described in detail. It is to be noted that, as a prior art display apparatus of the type just described, a display apparatus is disclosed, for example, in Japanese Patent Laid-Open No. 332434/1994.
In battery-driven portable electronic equipments, an LCD (Liquid Crystal Display) unit is used frequently as a display device. Also a self-luminous display unit is often used because it has such a characteristic that it is good in visual observability. The self-luminous display unit is usually used, in equipments wherein a battery is used as a power supply, in combination with a step-up type DC/DC converter.
Since the self-luminous display unit is of the current driven type and each of pixels thereof directly emits light, it exhibits much higher current consumption than that of the other display devices of the voltage driven type such as an LCD. Further, since the current consumption of the self-luminous display unit increases in proportion to the number of pixels from which light is emitted, a great difference appears in the current consumption depending upon whether all pixels emit light or some of the pixels emit light.
Therefore, the circuit of a DC/DC converter for a power supply for a self-luminous display unit is designed and the inductance value of a coil is selected so that the DC/DC converter may exhibit a maximum efficiency when the current consumption is maximum, that is, when all pixels emit light. This gives rise to a problem that, when only some of the pixels emit light in a standby state of a portable telephone set or in a like case, the efficiency of the DC/DC converter decreases while useless power consumption increases and the standby time decreases as much.
It is an object of the present invention to provide a DC/DC converter for a self-luminous display unit by which the efficiency can always be kept high with a simple configuration irrespective of the lighting condition of the self-luminous display unit and accordingly of whether or not the current consumption of the self-luminous display unit is high and a battery for an electronic equipment such as a portable telephone set in which the self-luminous display unit is incorporated can be used for a time longer than ever.
It is an object of the present invention to provide a self-luminous display apparatus wherein a DC/DC converter used as a power supply can always operate in a high efficiency with a simple configuration irrespective of the lighting condition of the self-luminous display apparatus and accordingly of whether or not the current consumption is high and a battery for an electronic equipment such as a portable telephone set in which the self-luminous display apparatus is incorporated can be used for a time longer than ever.
In order to attain the objects described above, according to an aspect of the present invention, there is provided a DC/DC converter for being used as a power supply for a self-luminous display unit which includes a plurality of self-luminous pixels, comprising a plurality of choke coils connected in parallel to each other to a DC power supply, a rectifying diode connected commonly to the choke coils, a switching element connected to the choke coils and switched ON/OFF in a predetermined period, a control circuit for controlling the ON/OFF operation of the switching element to control an output voltage of the DC/DC converter, and changeover means for selectively changing over the choke coils such that only one of the choke coils which has an inductance value corresponding to the output current of the DC/DC converter is effectively connected to the rectifying diode. With the DC/DC converter, the efficiency can always be kept high with the simple configuration of selectively changing over the choke coils irrespective of the lighting condition of the self-luminous display unit and accordingly of whether or not the current consumption of the self-luminous display unit is high, and accordingly, a battery for an electronic equipment such as a portable telephone set in which the self-luminous display unit is incorporated can be used for a time longer than ever.
According to another aspect of the present invention, there is provided a DC/DC converter for being used as a power supply for a self-luminous display unit which includes a plurality of self-luminous pixels, comprising a series circuit of a changeover switch connected at an input terminal thereof to a DC power supply and serving as changeover means which is controlled for changing over with a changeover control signal and a plurality of choke coils having first terminals connected to corresponding ones of changeover contacts of the changeover switch and second terminals connected to each other and having inductance values different from each other, a transistor connected at a collector terminal and an emitter terminal thereof between a node between the choke coils of the series circuit and the ground, a rectifying diode having an anode terminal connected to the node between the choke coils of the series circuit, a smoothing capacitor connected between a cathode of the rectifying diode, which serves as an output terminal, and the ground, an output voltage control circuit connected to the cathode of the rectifying diode for receiving a voltage of the output terminal, and an oscillation circuit for receiving a control output from the output voltage control circuit and being connected to a base terminal of the transistor in accordance with the control output of the output voltage control circuit to vary a duty ratio of an output thereof.
In both of the DC/DC converters, where the self-luminous display unit is incorporated in a portable telephone set, preferably the changeover means is controlled such that, in response to a standby operation of the portable telephone set wherein the number of those pixels of the self-luminous display unit which emit light is set small, an optimum one of the choke coils with which a high efficiency operation suitable for the number of those pixels which currently emit light is obtained is selected. This simplifies the selection or changeover control of the choke coils.
Meanwhile, the self-luminous pixels of the self-luminous display unit may be disposed in rows and columns and driven by a column driver circuit and a row driver circuit, which are controlled by a display controller, to emit light.
According to a further aspect of the present invention, there is provided a self-luminous display apparatus, comprising a self-luminous display unit including a plurality of self-luminous pixels, a DC/DC converter serving as a power supply for supplying power to the self-luminous display unit and including a plurality of choke coils connected in parallel to each other to a DC power supply, a rectifying diode connected commonly to the choke coils, a switching element connected to the choke coils and switched ON/OFF in a predetermined period, a control circuit for controlling the ON/OFF operation of the switching element to control an output voltage of the DC/DC converter, and changeover means for selectively changing over the choke coils such that only one of the choke coils which has an inductance value corresponding to the output current of the DC/DC converter is effectively connected to the rectifying diode, and a display controller for controlling display contents of the self-luminous display unit, the display controller further controlling the changeover means so as to select only an optimum one of the choke coils of the DC/DC converter with which a high efficiency operation is obtained for current display contents.
According to a still further aspect of the present invention, there is provided a self-luminous display apparatus, comprising a self-luminous display unit including a plurality of self-luminous pixels, a DC/DC converter serving as a power supply for supplying power to the self-luminous display unit and including a series circuit of a changeover switch connected at an input terminal thereof to a DC power supply and serving as changeover means which is controlled for changing over with a changeover control signal and a plurality of choke coils having first terminals connected to corresponding ones of changeover contacts of the changeover switch and second terminals connected to each other and having inductance values different from each other, a transistor connected at a collector terminal and an emitter terminal thereof between a node between the choke coils of the series circuit and the ground, a rectifying diode having an anode terminal connected to the node between the choke coils of the series circuit, a smoothing capacitor connected between a cathode of the rectifying diode, which serves as an output terminal, and the ground, an output voltage control circuit connected to the cathode of the rectifying diode for receiving a voltage of the output terminal, and an oscillation circuit for receiving a control output from the output voltage control circuit and being connected to a base terminal of the transistor in accordance with the control output of the output voltage control circuit to vary a duty ratio of an output thereof, and a display controller for controlling display contents of the self-luminous display unit, the display controller further controlling the changeover means so as to select only an optimum one of the choke coils of the DC/DC converter with which a high efficiency operation is obtained for current display contents.
In both of the self-luminous display apparatus, where it is incorporated in a portable telephone set, preferably the changeover means is controlled such that, in response to a standby operation of the portable telephone set wherein the number of those pixels of the self-luminous display unit which emit light is set small, an optimum one of the choke coils with which a high efficiency operation suitable for the number of those pixels which currently emit light is obtained is selected. This simplifies the selection or changeover control of the choke coils.
Meanwhile, preferably the display controller selects, in response to the number of those pixels of the self-luminous display unit which are to emit light, only one of the choke coils with which a high efficiency operation is obtained for the number of those pixels which currently emit light.
Further, the self -luminous pixels of the self-luminous display unit may be disposed in rows and columns and driven by a column driver circuit and a row driver circuit, which are controlled by the display controller, to emit light.
With the DC/DC converters and the self-luminous display apparatus, when the self-luminous display unit is used, a choke coil to be used in the DC/DC converter which supplies power of a stepped up voltage to a driver circuit of the self-luminous display unit is selectively used from among a plurality of choke coils having different inductance values from each other in accordance with output current of the self-luminous display unit. Consequently, the efficiency of the DC/DC converter in any display state of the self-luminous display unit can be optimized, and the current consumption in accordance with display contents can be optimized.
Where the changeover of the choke coils is performed by the display control section in response to the number of light emitting pixels or the like, then the efficiency of the DC/DC converter can be optimized with a simple configuration without requiring a detection circuit for output current and so forth.
Also where the changeover of the choke coils is performed based on an operation state of the electronic equipment, the efficiency of the DC/DC converter can be optimized similarly with a simple configuration which does not require a detection circuit for output current or the like.
The above and other objects, features and advantages of the present invention will become apparent from the following description and the appended claims, taken in conjunction with the accompanying drawings in which like parts or elements are denoted by like reference symbols.